This invention relates to a photonic crystal for use as a ultrasmall optical circuit device, and more particularly, to a two-dimensional photonic crystal having a two-dimensional periodic distribution of refractive index. It further relates to a waveguide having a light or electromagnetic radiation outlet/inlet port created by introducing a line defect and a point defect in such photonic crystal, and a wavelength demultiplexer.
With the recent advance of wavelength multiplexing communication systems, wavelength demultiplexers, multiplexers and filters become more important.
The optical branching/inserting device for wavelength multiplexing communication systems, also known as optical add/drop multiplexing device, has a function of taking a signal of a certain channel out of multiplexed signals or add the same to an empty channel. General constructions include array waveguide diffraction grating and fiber grating types. The array waveguide diffraction grating is a kind of diffraction grating having an array of a plurality of optical waveguides of different length in which the difference in length between waveguides creates a wavelength-dependent slope of wavefront so that upon input of wavelength-multiplexed light, the light is demultiplexed in terms of wavelength into different waveguides to produce outputs (see Journal of IEICE, pp. 746-749, 1999, for example). In the fiber grating type, only signals of a specific wavelength are taken out of the drop port or introduced from the add port by Bragg reflection at the fiber grating.
In the prior art wavelength multiplexers/demultiplexers of the array waveguide diffraction grating type, however, the radius of curvature must be kept significantly large in order to reduce a bend loss, resulting in a very large device size.
Many proposals were then made based on the concept of forming an ultrasmall optical multiplexer/demultiplexer using photonic crystal. These proposals are described in, for example, Applied Physics Letters, vol. 75, pp. 3739-3741, 1999 (Reference 1) and Physical Review Letters, vol. 80, pp. 960-963, 1998 (Reference 2).
The photonic crystal is a crystal having a periodic distribution of refractive index therein, which enables to establish novel optical characteristics using an artificial periodic structure.
One of the important features of the photonic crystal is the presence of a photonic bandgap. In photonic crystal having a three-dimensional periodicity (referred to as a 3D photonic crystal, hereinafter), a full bandgap that prohibits propagation of light in all directions can be formed. This enables local confinement of light, control of spontaneous emission light, and formation of a waveguide by the introduction of a line defect, indicating a possibility to realize an ultrasmall optical circuit.
Reference 1 suggests that an ultrasmall light demultiplexer can be formed by branching a waveguide formed by introducing a line defect into a 3D photonic crystal, but does not illustrate any specific structure.
Active studies have been made on a photonic crystal having a two-dimensional periodic structure (referred to as a 2D photonic crystal, hereinafter), because its fabrication is relatively easy. Reference 2 describes the analytic results of a demultiplexer using a branched waveguide.
A refractive index periodicity structure of 2D photonic crystal is formed by arranging cylindrical holes in a high refractive index material in a square or triangular lattice pattern. Alternatively, it is formed by arranging cylinders of a high refractive index material in a low refractive index material in a square lattice pattern. Photonic bandgaps are formed from these periodicity structures whereby the propagation of in-plane light is controlled. By introducing a line defect into this periodic structure, a waveguide can be created. See, for example, Physical Review Letters, vol. 77, pp. 3787-3790, 1996, and Reference 2.
Reference 2 relates to the array of cylinders of a high refractive index material in a square lattice pattern. It is noted that although the propagation of light in the in-plane direction can be controlled by a bandgap as previously described, the propagation of light in upward and downward directions cannot be controlled by the periodic structure. Analysis is thus made on a straight waveguide and a 90xc2x0 bend branch configuration and branch configuration on the assumption that the height is infinite.
However, since it is impossible for an actual device to have an infinite height, light must be confined within a finite height.
On the other hand, where cylindrical holes are formed in a high refractive index material, a waveguide can be created by forming the high refractive index material as a slab, and providing low refractive index layers above and below the slab so as to confine light by total reflection.
However, no research has been made on multiplexers and demultiplexers of such a structure. Also, no research has been made on the 90xc2x0 bend branch configuration and branch configuration of guiding light propagating in the in-plane direction to the orthogonal direction or guiding light from the orthogonal direction to the in-plane direction.
Optical multiplexers and demultiplexers using a super-prism based on self-organized 3D crystal have also been studied. See, for example, Applied Physics Letters, vol. 74, pp. 1212-1214, 1999 and 0 plus E, December 1999, pp. 1560-1565. They are not combined with waveguides, and only the function of an independent device is investigated.
If a photonic crystal waveguide is able to deliver a light output with wavelength selectivity in a certain wavelength region or receive a light input with wavelength selectivity, it becomes possible to realize an optical circuit having a light demultiplexing/multiplexing function of much smaller size than conventional devices. Also, if light or electromagnetic radiation in a 2D photonic crystal waveguide can be guided to the orthogonal direction, a steric light or electromagnetic radiation circuit can be obtained.
An object of the invention is to provide a construction capable of guiding or receiving light or electromagnetic radiation propagating through a 2D photonic crystal waveguide in a direction orthogonal to the plane thereof, the construction being effective for forming a light or electromagnetic radiation waveguide or a light or electromagnetic radiation multiplexer/demultiplexer.
The above and other objects are attained by the invention which is defined below.
(1) A two-dimensional photonic crystal waveguide comprising
a two-dimensional photonic crystal structure based on a slab formed of a material having a higher refractive index than air, in which a material having a lower refractive index than the slab material is periodically arrayed to provide a refractive index distribution,
a photonic crystal waveguide created by forming a line defect in the periodic array of photonic crystal, the line defect functioning as a waveguide, and
at least one point defect disposed adjacent the photonic crystal waveguide to act as a disorder in the periodic array of photonic crystal,
wherein the point defect functions as a light or electromagnetic radiation outlet/inlet port for trapping light or electromagnetic radiation of a selected wavelength among light or electromagnetic radiation propagating through the waveguide and radiating it, or trapping light or electromagnetic radiation of a selected wavelength from without the waveguide and introducing it into the waveguide.
(2) The two-dimensional photonic crystal waveguide of (1) wherein the light or electromagnetic radiation outlet/inlet port is to radiate or introduce the light or electromagnetic radiation propagating in a direction orthogonal to the slab surface.
(3) The two-dimensional photonic crystal waveguide of (1) or (2) wherein the wavelength of light or electromagnetic radiation radiated or introduced by the point defect differs depending on the shape of the point defect.
(4) The two-dimensional photonic crystal waveguide of any one of (1) to (3) wherein the array of the lower refractive index material is formed by filling cylindrical holes in the slab with the lower refractive index material.
(5) The two-dimensional photonic crystal waveguide of any one of (1) to (4) wherein the array of the lower refractive index material is a triangular lattice array.
(6) The two-dimensional photonic crystal waveguide of any one of (1) to (5) wherein the point defect is configured so as to be asymmetric on opposite sides with respect to the slab surface.
(7) The two-dimensional photonic crystal waveguide of any one of (1) to (6) wherein the slab material has a refractive index of at least 2.0.
(8) The two-dimensional photonic crystal waveguide of (7) wherein the slab material is an inorganic material containing at least one element selected from the group consisting of In, Ga, Al, Sb, As, Ge, Si, P, N, and O or an organic material.
(9) The two-dimensional photonic crystal waveguide of any one of (1) to (8) wherein the lower refractive index material is air.
(10) A photonic crystal wavelength demultiplexer comprising the two-dimensional photonic crystal waveguide of any one of (1) to (9).
(11) The photonic crystal wavelength demultiplexer of (10) wherein there are included a plurality of point defects and wavelength of light or electromagnetic radiation radiated or trapped by each point defect differs.
(12) The photonic crystal wavelength demultiplexer of (10) or (11), further comprising an optical fiber disposed in proximity to the point defect.
(13) The photonic crystal wavelength demultiplexer of (10) or (11), further comprising a semiconductor device having a photoelectric conversion function disposed in proximity to the point defect.
In the 2D photonic crystal waveguide of the invention comprising a 2D photonic crystal structure based on a slab formed of a material having a higher refractive index than air, in which a material having a lower refractive index than the slab material is periodically arrayed to provide a refractive index distribution, a photonic crystal waveguide is created by forming a line defect, which functions as a waveguide, in the periodic array of photonic crystal, and at least one point defect is disposed adjacent the photonic crystal waveguide to act as a disorder in the periodic array of photonic crystal. The point defect functions as a light or electromagnetic radiation outlet/inlet port for trapping light or electromagnetic radiation of a selected wavelength among light or electromagnetic radiation propagating through the waveguide and radiating it, or trapping light or electromagnetic radiation of a selected wavelength from the exterior and introducing it into the waveguide.
The waveguide and the point defect according to the invention can transport, receive and deliver not only light, but also electromagnetic radiation having a character approximate to light.
Where a point defect is formed in proximity to the waveguide formed by the line defect, introduction and removal of light or electromagnetic radiation into and out of the point defect becomes possible so that a light or electromagnetic radiation inlet/outlet port is available. The point defect becomes a disordered site in the 2D photonic crystal structure based on a slab formed of a material having a higher refractive index than air, in which a material having a lower refractive index than the slab material is periodically arrayed to provide a refractive index distribution.